Kesterite Deposited by Spray Pyrolysis for Solar Cell Applications

Abstract

En esta tesis se demuestra el uso de un sistema de spray pyrolysis utilizado para sintetizar kesterita de azufre puro (CZTS) un material que representa un reto tecnológico y científico en el campo de las celdas solares de películas delgadas. La síntesis de este material es llevada a cabo en un sistema de spray en atmosfera controlada en el marco de los parámetros del sistema y de la solución; evitando el uso de reactivos altamente peligrosos utilizando en su caso agua y alcoholes. Se demuestra la síntesis de materiales del tipo CZTSSe después de un proceso de selenización; las celdas solares resultantes muestran las posibilidades del material y del sistema.

Solar cells generate electrical power by direct conversion of solar radiation into electricity using semiconductors. Once produced, the solar cells do not require the use of water; operate in silence and can be easily installed almost everywhere, as solar panels with low technological risk. In this thesis new photovoltaic materials and solar cells are investigated. From the beginning of the semiconductor era, silicon has been present; the semiconductor theory improved with the silicon technology, almost taking the idealized models to reality in within silicon. In the recent years plenty of new natural and artificially produced materials have seen the light; some of them are still waiting to be understood and explained by a new theory that has to be experimentally proved right. The clue for a better and faster progress is to work in a multidisciplinary frame; as this thesis shows, all research and knowledge has to have future protections and possibilities of been used in the benefit of our society. Today, the photovoltaic technology (PV) based on silicon solar cells dominates the market. The thin film PV such as GaAs, Cu(In,Ga)Se2 (GIGS) and CdTe have reached power conversion efficiencies above 20% which makes them industrially interesting despite the use of scare and/or very toxic elements. Researchers and investors are expectant for a new stable, eco-friendly, inexpensive, fast and easy to produce material that could be used in a big scale and long term for photovoltaic terrestrial applications. Some years ago it was believed that this ideal material was near to be confirmed when the reflectors were on a new chalcogenide material: Cu2ZnSn(SxSe1-x)4 (CZTSSe) called kesterite after its crystal structure. This semiconductor is very attractive due to its constituent elements, semiconductor properties. Its similarity with GIGS and compatibility with the already existing industrial processes made possible its rapid power conversion efficiency rise as absorber material in thin film solar cells. Through the years kesterite has been found to be a challenging material due to the energetically feasible mixture of stannite and kesterite structures, the high probability of defects, and the narrowness of the optimum compositional region (compared with that of chalcopyrites) that eases the formation of secondary phases limiting the efficiency of the solar cells. Recently, CZTSSe thin film solar cells with certified efficiency of 12.6% were produced by IBM; synthesized by spin-coating using a hydrazine-based pure solution approach on a soda-lime glass (SLG) Mo coated substrate. The use of hydrazine is the key for the record as well its mayor drawback however it demonstrates the robustness of the solution-based techniques. In this thesis the use of a cool-wall vertical pneumatic spray pyrolysis system (SP) is demonstrated as a synthesis technique of CZTS kesterite thin films from water and alcohol-based precursor solutions containing metal salts and thiourea. In the course of this thesis, the possibilities and limitations of this synthesis technique and the resulting films are explored in the frame of system- and solution-related parameters. The SP system used in this thesis is sophisticate and advantageous; is able of reproduce the open-air conditions used in typical spray systems but also it is capable of grow films by spraying in an oxygen-free atmospheres such as Ar or Ar-H2 or any other. It was completely new spray approach by the year of the publication of our first repot (2013) with a 0.5% efficient working solar cell. A remarkable efficiency value by the time of publication if considered the combo challenge: material + deposition technique. In this thesis, air, Ar and Ar-H2 were used as carrier gas and atmosphere, where the so sprayed films were studied in combination with other system parameters (solution flux, time of spraying, substrate temperature, etc.) and some solution related parameters (solvent, metal precursors concentration, solution stability, etc.). To obtain device grade films, the sprayed kesterite ought to be annealed; this annealing process is also subject of study in this thesis. One step annealing at high temperature (580°C) at room pressure in S-containing reactive atmosphere was optimized for the CZTS-based thin films with efficiencies of 1.4%. To synthesize CZTSSe films, different annealing approaches were tried; one step room pressure annealing (at 550°C) in Se- containing reactive atmosphere probed to be the optimum for sprayed kesterite from methanol-based precursor solutions for solar cells with the highest conversion efficiency of 1.9% obtained in this thesis. The results showed here open many new possibilities for the use of spray systems for the synthesis of PV quality materials for solar cells applications.